Transformer Current Calculator (Full-Load Amps)

How to use the transformer current calculator

Enter the transformer rating in kVA, choose three-phase or single-phase and enter the primary and secondary line voltages. You get the full-load current on each side and the voltage ratio.

Transformer current formulas

Supply	Full-load current
Three-phase	I = kVA × 1,000 ÷ (√3 × VL)
Single-phase	I = kVA × 1,000 ÷ V

Full-load currents (three-phase)

kVA	11 kV side	433 V side	415 V side
25	1.3 A	33 A	35 A
63	3.3 A	84 A	88 A
100	5.2 A	133 A	139 A
160	8.4 A	213 A	223 A
250	13.1 A	333 A	348 A
315	16.5 A	420 A	438 A
400	21.0 A	533 A	556 A
500	26.2 A	667 A	696 A
630	33.1 A	840 A	876 A
1000	52.5 A	1333 A	1391 A
1600	84.0 A	2133 A	2226 A
2000	105.0 A	2667 A	2782 A
2500	131.2 A	3333 A	3478 A

A useful shortcut at 415–433 V three-phase: full-load current ≈ 1.33–1.39 × kVA.

Selecting protection

• LV breaker: rated at or just above the secondary full-load current.
• HV fuse or breaker: sized for full-load current while riding through magnetising inrush (often 8–12 × rated current for a few cycles).
• Cables and busbars: rated for full-load current with installation derating factors.

Loading

Distribution transformers are most efficient at about 40–60% load. Continuous overloading shortens insulation life; follow the manufacturer’s loading guide (IEC 60076-7).

Fault current

Short-circuit current depends on the transformer’s impedance. See the transformer short-circuit calculator.

Frequently asked questions

How do I calculate transformer full-load current?

Three-phase I = kVA × 1,000 ÷ (√3 × V); single-phase I = kVA × 1,000 ÷ V, using line voltage.

What is the full-load current of a 500 kVA, 11 kV/433 V transformer?

About 26.2 A on the 11 kV side and 667 A on the 433 V side.

Why is distribution secondary voltage 433 V, not 415 V?

The no-load secondary is set slightly higher to allow for voltage drop under load, so customers receive close to 415 V.

Does power factor affect transformer current?

Not the rating calculation — transformers are rated in kVA, so full-load current depends only on kVA and voltage.

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